Extended area acoustic impulse generator

ABSTRACT

An acoustic impulse generator for producing in a liquid body acoustic impulses useful, for example, in geophysical explorations. The generator includes a housing which defines an enclosed chamber having a flexible wall. Driving means in one operating condition cause the flexible wall to execute a forward stroke in the liquid body thereby storing potential energy in the liquid body. The driving means in another operating condition allow the flexible wall to execute a return stroke in a relatively short time interval thereby generating an acoustic impulse.

United States Patent Savit [54] EXTENDED AREA ACOUSTIC IMPULSE GENERATOR [72] Inventor: Carl H. Savlt, Houston, Tex.

[73] Assignee: Western Geophysical Company of America, Houston, Tex.

[22] Filed: July 23, 1969 [21] Appl. No.: 844,152

[52] US. Cl. ..l8l/0.5 H, 181/05 R, 340/14 [51] Int. Cl. ..G0lv 1/02 [58] Field ofSearch ..181/0.5 1C, 0.5 H; 340/14 [56] References Cited UNITED STATES PATENTS 3,233,694 2/1966 Roever ..18l/0.5 1C 3,277,437 10/1966 Bouyoucos.. ....l81/0.5 H 3,369,627 2/1968 Schempf I8l/0.5 H

[4 1 June 20, 1972 11/1969 Barry et al. ..l8l/0.5 8/1970 Lister ..l8l/0.5 EM

Primary Examiner-Benjamin A. Borchelt Assistant Examiner-H. A. Birmiel Attorney-Michael P. Breston, Alan C. Rose and Alfred B. Levine [57] ABSTRACT An acoustic impulse generator for producing in a liquid body acoustic impulses useful, for example, in geophysical explorations. The generator includes a housing which defines an enclosed chamber having a flexible wall. Driving means in one operating condition cause the flexible wall to execute a forward stroke in the liquid body thereby storing potential energy in the liquid body. The driving means in another operating condition allow the flexible wall to execute a return stroke in a relatively short time interval thereby generating an acoustic impulse.

1 Claim, 6 Drawing Figures PATENTEfJJuuzo I972 3. 670, 839

sum. 1 or 2 Carl H. Savit INVENTOR II I I 3 BY Michael PBreston ATTORNEY PATENTEDmzomz 3,670,839

SHEET 2 BF 2 FIG. 4

Carl H. Savit INVENTOR BY Michael Pfireston ATTORNEY EXTENDED AREA ACOUSTIC IMPULSE GENERATOR BACKGROUND OF THE INVENTION Acoustic impulse generators for producing in a liquid acoustic impulses are known in the art. US. Pat. No. 3,369,627 shows two adjacently positioned circular plates towed in'sea water. Driving means cause the lower plate to execute a forward stroke relative to the upper plate in a relatively short time interval thereby creating a cavity between the plates. The violent collapse of this cavity by the surrounding water generates an acoustic impulse which propagates throughout the body of water. Subsequent to the generation of the acoustic impulse, the driving means cause the lower plate to execute a relatively slow return stroke.

In such a generator, the power required to execute a very fast forward stroke against the ambient water pressure is very great. Such generators, therefore, require relatively large, high-power actuators. Moreover, since the bottom plate which executes the forward stroke is completely exposed to the open water at the time when the surrounding water rushes in to collapse the cavity, both plates rapidly experience structural fatigue.

U. S. Pat. No. 3,277,437 shows another type of acoustic generator submerged in a body of water which includes an enclosed chamber having a movable piston. Under the influence of a fluid-pressure-and-vacuum source the piston executes a forward stroke. The piston is then permitted to execute a return stroke thereby converting potential energy, stored in the liquid body during the forward stroke, into kinetic energy which makes available energy for the formation of an acoustic shock wave when the piston comes to rest. In this type of generator the acceleration of the piston during the return stroke is relatively limited.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a generator for generating high-power acoustic impulses when submerged in a liquid body. The generator comprises a housing which defines an enclosed chamber having a flexible wall. Driving means are coupled to the wall to cause the wall to execute a forward stroke in the liquid body. During the forward stroke of the wall potential energy is stored in the ambient liquid body. The forward stroke may be relatively slow. The potential energy stored in the liquid body is released upon command to cause the flexible wall to execute a return stroke in a relatively short time interval.

If the flexible wall is elastic and if the wall is stretched in the course of the forward stroke, potential energy will also be stored in the wall; thus providing additional acceleration to the wall during the return stroke. The greater the potential energy stored in the generators flexible wall, the greater will be the acceleration of the wall during the return stroke. Consequently, the wall can be made to move away from the liquid body into the enclosed chamber faster than the liquid body can follow the wall. A cavity is then formed between the wall and the surrounding liquid body. The surrounding liquid rushes in to fill this cavity, and in so doing a high-power acoustic impulse becomes generated and propagated throughout the liquid body.

In accordance with a specific embodiment of this invention there is provided an acoustic impulse generator having a housing which defines a chamber having a flexible wall. Driving means are coupled to the flexible wall.

The driving means in one operating condiu'on cause the flexible wall to execute a forward stroke against the ambient pressure exerted by the liquid body and, if the wall is elastic, against the elastic resistance offered by the wall. As a result during the forward stroke potential energy is stored in the liquid body and in the wall, if elastic. The driving means in another operating condition allow the wall to execute a return stroke.

In the case of the elastic wall, the acceleration of the wall can be made sufficiently high to create a cavity between the liquid body and the wall, i.e., the wall during the return stroke can be made to move away from the liquid body-faster than the liquid can follow the wall. The surrounding liquid th'en violently moves in to fill this cavity. A portion of the kinetic energy of the moving in liquid becomes converted into acoustic energy in the form of a relatively high-level, lowfrequency pulse. Thus the violent collapse of this cavity produces an acoustic impulse which propagates throughout the liquid medium.

In the case of the flexible but inelastic wall, the surrounding liquid remains'in contact with the wall during the return stroke and produces an acoustic impulse when the wall abruptly comes to rest.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows in perspective a preferred embodiment of the extended area acoustic impulse generator of this invention;

FIG. 2 is a view taken on line 2-2 of FIG. 1;

FIG. 3 is an enlarged detail view showing the manner of coupling the flexible wall;

FIG. 4 shows another manner of coupling the flexible wall;

FIG. 5 is a top view of another embodiment of this invention; and

FIG. 6 is a view taken on line 6-6 in FIG. 5.

High-power, relatively low-frequency acoustic impulse generators or radiators for generating acoustic impulses in a liquid body, as in sea or ocean water, are particularly useful for seismic prospecting. In the case of sub-bottom seismic exploration at sea, the acoustic impulses or signals of the greatest utility are those whose frequency lies in the range between 5 and 50 Hz or whose wave length in water lies between a thousand feet and feet. Consequently, most seismic impulse generators or radiators are, because of logistic considerations, small compared to a wave length.

It has been established through theoretical, analysis of the physical laws of acoustic radiation that the effectiveness of small (with respect to a wave length) acoustic generators is proportional to the square of their surface area. That is, as long as the linear dimensions of an acoustic impulse generator are small compared to the wave length of energy generated, the acoustic energy radiated to the far field will be approximately proportional to the square of the generators radiating surface area.

For a given amount of input power, therefore, it is more effective to move a large radiating-surface a small distance than to move a small area a relatively large distance. The radiated acoustic power will be proportional to the product of the area and the distance traveled by the radiating surface.

As a numerical example, it may be considered that if the dimensions of the radiating surface are 10 by 10 feet and if the stroke or movement of the radiating area is 1 inch, the generator will produce on the order of 16 times as much far field acoustic energy as a generator whose dimensions of the radiating surface are 2% by 2% feet and whose stroke is 16 inches. Both such generators would requ'ue the same input energy.

Referring now more specifically to the drawings, and particularly to FIGS. 1 through 3 thereof, there is shown an extended area acoustic impulse generator generally designated as 10. Generator 10 is shown completely submerged to a given depth in a liquid body such as sea water 12. The depth below the air/water interface at which generator 10 is positioned is sometimes herein referred to as the operating depth characterized by an operating ambient hydrostatic pressure.

Generator 10 may assume various geometric configurations. In the embodiment shown in FIG. 1, the generator I defines a chamber 14 having a flexible wall 16 fixedly and sealingly coupled to a rigid anvil wall 18, as by bolts 20 fastened to a retaining ring 22. Anvil 18 is preferably made of metal and the selection of its linear dimensions are determined by the design criteria above described. The edges of anvil 18 have a relatively greater thickness dimension than its center portion to add greater rigidity to the structure of generator 10 and to allow a greater displacement or stroke for the flexible wall 16. Wall 16 may be made of a suitable flexible material which may be a reinforced plastic or a natural material such as rubber. Wall 16 may be elastic or inelastic depending on the acceleration desired for wall 16 during its return stroke.

The impulse generator may be connected to a suitable fluid pressure and vacuum source means, not shown, for applying fluid pressure to and removing fluid pressure from chamber 14. Fluid pressure is applied to an inlet 24 coupled to a pressure line 26. The application of fluid pressure through inlet 24 into chamber 14 is controlled by a suitable valve 28 which may be pilot-operated by a fluid-pressure line 30. The fluid pressure fed to chamber 14 may be compressed air. It will be appreciated that the pressure source can be positioned on a vehicle or seismic boat and can be a compressor operated with diesel fuel to compress air which is freely available.

To evacuate chamber 14 there are provided one or more local fluid receivers or containers. Three such receivers 32, 34 and 36 are shown, respectively communicating with chamber 14 through outlets 38, 40 and 42. As can be seen from FIG. 1, receiver 36 communicates with chamber 14 through three outlets 42, chamber 34 through three outlets 40, and chamber 32 through three outlets 38. Outlets 38, 40 and 42 are controlled through pilot-operated valves 46, 48 and 50.

To operate valves 46, 48 and 50 substantially simultaneously there is provided a fluid pressure line 52, which may be controlled by a solenoid-operated valve (not shown), and connected to the fluid pressure source. Line 52 branches out into substantially equal-length branches 54, 56 and 58. The volume of receivers 32-36 is selected to provide for a very rapid evacuation of the fluid from chamber 14. Receivers 32, 34 and 36 are maintained continuously at a vacuum by a vacuum-operated conduit 60 having three branch lines 62, 64 and 66 coupled to suitable inlets 68, 70 and 72, respectively. As previously mentioned the pumping means which applies fluid pressure to chamber 14 may also be used to evacuate chamber 14. The fluid pressure and vacuum source forms no part of this invention. Generator 10 is normally attached to be towed by a vehicle such as a seismic boat (not shown) as by a pair of cables 74.

The operation of one complete cycle of generator 10 will be described by starting with the flexible wall 16 at its rest or contracted position as shown in FIG. 3. Chamber 14 is then evacuated and wall 16 is pressed by the outside ambient pressure against the anvil wall 18. Valve 28 is closed and valves 46, 48 and 50 are open.

Flexible wall 16 will execute it forward stroke by inflating chamber 14, by closing valves 46, 48 and 50 and by opening valve 28. Fluid pressure is then allowed to enter chamber 14 through inlet 24. The inflation of chamber 14 may have a time period which is relatively long compared to the duration of the evacuation of chamber 14. The movement of flexible wall 16 displaces a column of water in front of wall 16 thereby storing potential energy in the displaced water. This potential energy is proportional to the product of the ambient pressure and the volume of the water displaced.

If wall 16 is inelastic it will not stretch appreciably and no potential energy will be stored in wall 16. On the other hand, if wall 16 is elastic it will stretch by the application of sufiicient fluid pressure through line 26. The stretching of elastic wall 16 allows it to store potential energy. When chamber 14 is inflated as shown in FIG. 2, wall 16 is in its cocked or fire position.

To deflate chamber 14 very rapidly, valve 28 is closed and valves 46, 48 and 50 are simultaneously opened thereby opening outlets 38, 40 and 42 respectively. The fluid from chamber 14, which as previously mentioned may be compressed air, is rapidly evacuated into local vacuum receivers 32, 34 and 36,

and through line 60 is evacuated by the vacuum source. A relatively large volume vacuum receiver, positioned on the deck of the seismic boat, may be used in connection with the vacuum source.

In the case of the flexible but inelastic wall 16, the surrounding water remains in contact with wall 16 during it return stroke and produces an acoustic impulse when wall 16 abruptly comes to rest against the anvil wall 18. The acceleration of wall 16 is primarily determined by the potential energy stored in the volume of displaced water.

On the other hand, if flexible wall 16 is elastic and if wall 16 becomes stretched in the course of the forward stroke, potential energy is also stored in wall 16. The potential energy stored in wall 16 will provide additional acceleration to wall 16 during its return stroke. Consequently, wall 16 can be made to move away from the ambient water at a rate which is faster than the ambient water can follow wall 16. A cavity is then formed between wall 16 and the surrounding water body. The rushing in of the surrounding water to fill this cavity will generate a relatively high-power acoustic impulse which will be propagated throughout the water body.

In the modified wall coupling shown in FIG. 4, the resilient wall 16 is secured to the top face of end wall 18 by a plurality of bolted clamps 80.

Referring now to the embodiment shown in FIGS. 5 and 6, the anvil wall 82 has a curved cross-section, and wall 82 may be semi-elliptical or semi-spherical in shape. A high-pressure fluid line 84 communicates with inlet 86 through a solenoidoperated valve 88. High-pressure line 84 will inflate chamber 14 in a manner previously described. To evacuate chamber 14 there is provided a doughnut-shaped local receiver 90 communicating with chamber 14 through a number of outlet ports 92, each controlled by a valve 94 which may be pilot or solenoid-operated. The vacuum in local receiver 90 is pulled by a vacuum line 96 connected to the vacuum source on board the seismic boat.

The operation of the embodiment shown in FIG. 5 is in all respects similar to the operation described in connection with the embodiment shown in FIG. 1.

While the inflation of chamber 14 has been described in particular connection with a single pressure level applied through line 26, it will be appreciated by those skilled in the art that a train of pressure pulses of varying levels can be applied through line 26 from a suitably programmed fluid pressure source (not shown) to fill the volume of chamber 14.

What I claim is:

l. A generator for generating a high-power cavitation acoustic impulse when submerged in a liquid body, said generator being adapted to store potential energy, said generator comprising:

first means defining an enclosed chamber having an elastic wall at least about a portion of the periphery of said chamber;

vacuum-operated driving means adapted to move said elastic wall,

said driving means in one operating condition causing said elastic wall to execute a forward stroke in said liquid body thereby storing potential energy in said liquid body and in said elastic wall,

said driving means in another operating condition allowing said elastic wall to execute a return stroke in a short time interval thereby generating said acoustic impulse,

said time interval is sufficiently short to allow for the formation of a cavity between said wall and said liquid body, and

the filling of said cavity by the surrounding liquid body generates said acoustic impulse. 

1. A generator for generating a high-power cavitation acoustic impulse when Submerged in a liquid body, said generator being adapted to store potential energy, said generator comprising: first means defining an enclosed chamber having an elastic wall at least about a portion of the periphery of said chamber; vacuum-operated driving means adapted to move said elastic wall, said driving means in one operating condition causing said elastic wall to execute a forward stroke in said liquid body thereby storing potential energy in said liquid body and in said elastic wall, said driving means in another operating condition allowing said elastic wall to execute a return stroke in a short time interval thereby generating said acoustic impulse, said time interval is sufficiently short to allow for the formation of a cavity between said wall and said liquid body, and the filling of said cavity by the surrounding liquid body generates said acoustic impulse. 